Silicon carbide material for nuclear applications, precursor and method for forming same, and structures including the material

ABSTRACT

A precursor formulation of a silicon carbide material that includes a ceramic material and a boron-11 compound. The ceramic material may include silicon and carbon and, optionally, oxygen, nitrogen, titanium, zirconium, aluminum, or mixtures thereof. The boron-11 compound may be a boron-11 isotope of boron oxide, boron hydride, boron hydroxide, boron carbide, boron nitride, boron trichloride, boron trifluoride, boron metal, or mixtures thereof. A material for use in a nuclear reactor component is also disclosed, as are such components, as well as a method of producing the material.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.11/357,716, filed Feb. 16, 2006, entitled “SILICON CARBIDE MATERIAL FORNUCLEAR APPLICATIONS, PRECURSOR AND METHOD FOR FORMING SAME, ANDSTRUCTURES INCLUDING THE MATERIAL,” pending, the disclosure of which isincorporated herein in its entirety by reference.

FIELD OF THE INVENTION

The present invention relates to a stable silicon carbide (“SiC”)material for use in the nuclear industry, such as in components of anuclear reactor, as well as to such components. More specifically, thepresent invention relates to a SiC material that includes a boron-11(“¹¹B”) isotope, as well as to a precursor and method for forming theSiC material, and components including the SiC material.

BACKGROUND OF THE INVENTION

SiC fibers are known in the art for providing mechanical strength athigh temperatures to fibrous products, such as high temperatureinsulation, belting, gaskets, or curtains, or as reinforcements inplastic, ceramic, or metal matrices of high performance compositematerials. To provide good mechanical strength to these products ormaterials, the SiC fibers have a relatively high density (i.e., lowresidual porosity) and fine grain sizes. However, producing SiC fiberswith these properties is difficult because the SiC fibers typicallyundergo coarsening or growth of crystallites and pores during ahigh-temperature heat treatment.

Sintering aids have been used to improve densification of the SiC fibersand to prevent coarsening, allowing the SiC fibers to be fabricated withhigh density and fine grain sizes. These sintering aids are typicallycompounds of boron. As disclosed in U.S. Pat. No. 5,366,943 to Lipowitzet al., which is incorporated by reference in its entirety herein, theSiC fibers are formed by converting amorphous ceramic fibers topolycrystalline SiC fibers. The ceramic fibers are heated in thepresence of a sintering aid to produce the polycrystalline SiC fibers.The sintering aid is boron or a boron-containing compound, such as aboron oxide (“B₂O₃”). An example of SiC fibers prepared by this processis SYLRAMIC®, which is available from COI Ceramics, Inc. (San Diego,Calif., an affiliate of ATK Space Systems).

Another method of forming SiC fibers is by spinning a polycarbosilaneresin into green fibers, treating the green fibers with boron, andcuring and pyrolyzing the green fibers, as disclosed in U.S. Pat. No.5,071,600 to Deleeuw et al. Other methods of forming SiC fibers areknown, such as spinning organosilicon polymers into fibers, curing thefibers, and ceramifying the fibers at elevated temperatures. However,many of these methods undesirably introduce oxygen or nitrogen into theSiC fibers. When these SiC fibers are heated to temperatures above 1400°C., the oxygen or nitrogen is volatilized, causing weight loss,porosity, and decreased tensile strength in the SiC fibers. In additionto SiC fibers, SiC bodies are known to be formed by molding SiC powderand elemental carbon into a desired shape and heating the moldedstructure in a boron-containing environment.

While many methods of producing SiC fibers (or SiC bodies) are known,components or products formed from conventional SiC fibers, such asthose described above, are not suitable for use in nuclear applicationsdue to the boron compound used as the sintering aid. The boron compoundtypically includes boron-10 (“¹⁰B”). Boron has thirteen isotopes, two ofwhich, ¹⁰B and ¹¹B, are naturally occurring. The natural abundance of¹⁰B and ¹¹B is 19.9% and 80.1%, respectively. However, ¹⁰B is the mostcommercially available isotope because ¹⁰B is more easily extracted fromore than ¹¹B. ¹⁰B absorbs neutrons and is used in control rods ofnuclear reactors, as a shield against nuclear radiation, and ininstruments for detecting neutrons. However, ¹⁰B is unstable andundergoes fission when irradiated, producing a gamma ray, an alphaparticle, and a lithium ion. Therefore, when a component formed fromconventional SiC fibers is irradiated, the boron compound undergoesfission, which is accompanied by outgassing and degradation of the SiCfibers or the SiC bodies. As such, conventional SiC fibers are notsuitable for use in a component to be used in the nuclear industry, suchas in a nuclear reactor.

It would be desirable to produce SiC fibers or SiC bodies that are morestable to irradiation for use in components to be used in the nuclearindustry. For instance, it would be desirable to produce SiC fibers orSiC bodies that are useful in nuclear applications without outgassing ordegradation.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to a precursor formulation of a SiCmaterial that includes a ceramic material and a boron-11 compound. Asused herein, the term “SiC material” refers to SiC fibers, SiC bodies,or other forms of SiC ceramics, such as monolithic SiC, SiC coatings,SiC thin substrates, or porous SiC ceramics. The ceramic material mayinclude silicon and carbon and, optionally, oxygen, nitrogen, titanium,aluminum, zirconium, or mixtures thereof. For the sake of example only,the ceramic material may include SiC fibers, silicon oxycarbide fibers,silicon carbon nitride fibers, silicon oxycarbonitride fibers,polytitanocarbosilane fibers, or mixtures thereof. The boron-11 compoundmay be a boron-11 isotope of boron oxide, boron hydride, boronhydroxide, boron carbide, boron nitride, boron trichloride, borontrifluoride, boron metal, or mixtures thereof. The boron-11 compound mayaccount for less than or equal to approximately 2% by weight (“wt %”) ofa total weight of the precursor formulation. Thus, while the presentinvention is referred to for the sake of convenience in the singular as“a” precursor and “a” SiC material, it will be appreciated that a numberof different SiC materials and precursors that may be used to form avariety of materials are encompassed by the present invention.

The present invention also relates to a material for use in a nuclearreactor component. The material includes a SiC material and a boron-11compound. The SiC material may be SiC fibers, a SiC body, a SiC ceramic,a SiC coating, a SiC thin substrate, or a porous SiC ceramic. Theboron-11 compound may be one of the compounds previously described. Theboron-11 compound may account for from approximately 0.1 wt % of a totalweight of the material to approximately 4 wt % of the total weight ofthe material. A layer of boron nitride that includes the ¹¹B isotope(“¹¹BN”) may, optionally, be present on a surface of the material.

The present invention also relates to a method of producing a SiCmaterial by converting a ceramic material to a SiC material in thepresence of a boron-11 compound. The ceramic material may be convertedby heating the ceramic material in an environment that includes theboron-11 compound. The ceramic material and the boron-11 compound mayinclude one of the compounds previously described. Alternatively, theboron-11 compound may be formed by reacting a boron-11 containingmaterial with an oxidizing agent in situ. The boron-11 containingmaterial may be selected from the group consisting of boron carbide,boron, boron suboxide, and mixtures thereof and the oxidizing agent maybe selected from the group consisting of carbon dioxide, carbonmonoxide, oxygen, and mixtures thereof. As with the precursor andmaterial of the present invention, the method may be varied within thescope of the invention.

The ceramic material and the boron-11 compound may be heated to atemperature that is greater than approximately 1200° C., such as fromapproximately 1200° C. to approximately 1400° C., for an amount of timesufficient for the boron-11 compound to vaporize and diffuse into theceramic material and for volatile by-products to be released. Thesilicon carbide material may, optionally, be exposed to a nitrogenatmosphere to form a layer of ¹¹BN on a surface of the silicon carbidematerial.

The present invention also encompasses structures formed at least inpart of a material including an SiC material and a boron-11 compound,which structures may be exposed to nuclear radiation without outgassingor degradation and, so, are suitable for use in nuclear applications. Byway of example only, such structures include components for nuclearreactors, such as control rods, control rod guides, fuel cladding, coresupport pedestals, reactor core blocks, upper core gas plenum, interiorinsulation covers, hot ducts, heat exchangers, and combinations thereof.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

While the specification concludes with claims particularly pointing outand distinctly claiming that which is regarded as the present invention,the advantages of this invention may be more readily ascertained fromthe following description of the invention when read in conjunction withthe accompanying drawings in which:

FIG. 1 is a schematic illustration of a very high temperature nuclearreactor;

FIG. 2 is a schematic illustration of a control rod;

FIG. 3 illustrates an embodiment of a heat exchanger formed from the¹¹B—SiC material of the present invention; and

FIG. 4 illustrates an embodiment of fuel cladding formed from the¹¹B—SiC material of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

A SiC material that is stable to irradiation is disclosed. A precursorformulation of the SiC material includes a ceramic material and at leastone boron compound having a ¹¹B isotope (referred to herein as the “¹¹Bcompound”). Since ¹¹B is a stable isotope, the SiC material having the¹¹B compound (referred to herein as the “¹¹B—SiC material”) may be usedto produce components stable to irradiation. These components may beused in the nuclear industry, such as in nuclear fission reactors andfusion reactors. An example of a fission reactor includes, but is notlimited to, a GEN IV Very High Temperature Reactor. An example of afusion reactor includes, but is not limited to, a Tokamak reactor. Byusing ¹¹B in the boron compound, the ¹¹B—SiC material does not undergofission when irradiated. In addition, no outgassing or degradationoccurs when the component is irradiated.

The precursor formulation of the ¹¹B—SiC material includes the ceramicmaterial and the ¹¹B compound. Minor amounts of additional ingredientsor additives may also be present in the precursor formulation, such asto improve processability of the precursor formulation or performance ofthe ¹¹B—SiC material. However, these ingredients or additives are notneeded to provide the desired stability of the ¹¹B—SiC material tonuclear environments. The ceramic material in the precursor formulationmay be converted to the ¹¹B—SiC material by heating at a sufficienttemperature in the presence of the ¹¹B compound. The ceramic materialmay include silicon and carbon, which are present in near stoichiometricor moderately carbon-rich amounts. As used herein, the phrase“moderately carbon-rich” refers to a carbon content of less than orequal to approximately 2%. In one embodiment, the silicon and carbon arepresent in near stoichiometric amounts. The ceramic material may beamorphous or microcrystalline ceramic fibers that include sufficientsilicon and carbon to form a ¹¹B—SiC material that includesstoichiometric amounts of silicon and carbon or is carbon-rich. As usedherein, the phrase “carbon-rich” refers to a carbon content of greaterthan approximately 2%. The ceramic fibers may also include oxygen (“O”),nitrogen (“N”), titanium (“Ti”), aluminum (“Al”), zirconium (“Zr”), ormixtures thereof. If present, these elements may volatilize out of the¹¹B—SiC material during subsequent processing or remain in the ¹¹B—SiCmaterial without affecting its integrity or properties. Other elementsmay also be present in the ceramic fibers as long as the elements arevolatilized or remain in the ¹¹B—SiC material without affecting itsintegrity and properties. As described in more detail below, oxygenpresent in the ceramic material may, optionally, be removed(deoxygenated) before converting the ceramic material into the ¹¹B—SiCmaterial.

Methods of manufacturing the ceramic material used in the precursorformulation are known in the art and, therefore, are not discussed indetail herein. For instance, organosilicon polymers (with or withoutceramic powder additives) may be spun into fibers, and the fibers cured(infusibilized) and pyrolyzed to form the ceramic material. In addition,sol-gel processing techniques or chemical vapor deposition techniquesmay be used to produce the ceramic material. The ceramic material usedin the precursor formulation may have any length or any form desired.For instance, if fibers are used, the ceramic fibers may besubstantially continuous and may be used as either single strands (or 1or many filaments (tows)) or are aligned unidirectionally (e.g., tapes),woven as a two-dimensional fabric or shaped as a three-dimensionalperform.

Examples of ceramic fibers that may be used include, but are not limitedto, SiC fibers, silicon oxycarbide (“SiOC”) fibers, silicon carbonnitride (“SiCN”) fibers, silicon oxycarbonitride (“SiCON”) fibers, orpolytitanocarbosilane fibers (“SiCOTi”). In addition, mixtures of theseceramic fibers may be used. Such ceramic fibers are known in the art andare commercially available from various sources. For instance, SiOCfibers having a diameter in the range of from approximately 10 μm toapproximately 20 μm are manufactured by Nippon Carbon Co. (Tokyo, Japan)and are sold under the NICALON® tradename (e.g., Ceramic Grade (CG),High Volume Resistivity (HVR)). SiCOTi fibers having a diameter in therange of from approximately 8 μm to approximately 12 μm are manufacturedby Ube Industries, Ltd. (Yamaguchi, Japan) and are sold under theTYRANNO® tradename. Experimental ceramic fibers may also be used, suchas SiCON fibers having a diameter in the range of from approximately 6μm to approximately 10 μm and SiCON fibers having a diameter in therange of from approximately 10 μm to approximately 15 μm (produced byDow Corning (Midland, Mich.) and designated as “MPDZ”). In oneembodiment, the ceramic material includes SiOC fibers. In anotherembodiment, the ceramic material includes SiCOTi fibers, such asTYRANNO® Lox M fibers.

The ¹¹B compound in the precursor formulation may function as avolatile, sintering aid. The ¹¹B compound in the precursor formulationmay initially melt and then volatilize to form a vapor. To achieve thedesired properties in the ¹¹B—SiC material, an excess amount of the ¹¹Bcompound may be used relative to the amount of the ceramic material. Asexplained below, a small amount of the ¹¹B compound may be present inthe ¹¹B—SiC material after processing of the precursor formulation toform the ¹¹B—SiC material. The ¹¹B compound may have a significant vaporpressure at and above a temperature at which the ceramic material beginsto decompose or densify. For the sake of example only, for SiCO ceramicfibers, the decomposition temperature may be as low as approximately1200° C. for slow temperature heating (ramp) rates (e.g., <1° C./minute)or may range from approximately 1400° C. to approximately 1500° C. forheating rates of several degrees per minute or more. The ¹¹B compoundmay be a solid, a liquid, or a gas at room temperature.

The boron-11 compound may be a ¹¹B isotope of boron oxide (“¹¹B₂O₃”),boron hydride (“¹¹B₂H₆”), boron hydroxide (“¹¹B(OH)₃”), boron carbide(“¹¹B₄C”), boron nitride (“¹¹BN”) boron chloride (“¹¹BCl₃”), boronfluoride (“¹¹BF₃”), boron metal, or mixtures thereof. In one embodiment,the ¹¹B compound is ¹¹B₂O₃. Compounds that include the ¹¹B isotope arecommercially available from various sources. For instance, ¹¹B₂O₃ isavailable from Sigma-Aldrich Co. (St. Louis, Mo.) or EaglePicher Inc.(Phoenix, Ariz.). In addition, a mixture of two or more ¹¹B compoundsmay be used in the precursor formulation.

The ¹¹B compound may be present at less than or equal to approximately 2wt % of a total weight of the precursor formulation, such as fromapproximately 0.5 wt % to approximately 1.5 wt % of the total weight ofthe precursor formulation. The amount of ¹¹B compound used in theprecursor formulation may be adjusted as long as crystallite growth andporosity are minimized such that the strength of the ¹¹B—SiC materialremains in an acceptable range. The remainder of the precursorformulation may include the ceramic material and any optionalingredients. As such, the ceramic material may account for greater thanor equal to approximately 98 wt % of the total weight of the precursorformulation.

Alternatively, the ¹¹B compound may be formed in situ by reacting a¹¹B-containing material with an oxidizing agent, as described in U.S.Pat. No. 6,261,509 to Barnard et al., which is incorporated by referencein its entirety herein. For instance, the ¹¹B-containing material andthe oxidizing agent may be reacted at a temperature that ranges fromapproximately 1300° C. to approximately 1600° C., producing ¹¹B₂O₃vapor. The ¹¹B-containing material may include, but is not limited to,boron carbide (“B₄C”), boron, or boron suboxide (“B₆O”). The oxidizingagent may include, but is not limited to, carbon dioxide (“CO₂”), carbonmonoxide (“CO”), oxygen (“O₂”), or mixtures thereof. When the¹¹B-containing material and oxidizing agent react, the oxidizing agentis the rate-limiting reagent. Therefore, the concentration of theoxidizing agent may be adjusted to control the concentration of vaporous¹¹B₂O₃ produced. In addition, the rate of addition of the oxidizingagent may be adjusted to control the production of vaporous ¹¹B₂O₃. Theamount of oxidizing agent reacted with the ¹¹B-containing material maybe sufficient to provide a ¹¹B₂O₃ concentration sufficient to produce aminimum of 0.1 wt % of the ¹¹B compound in the resulting ¹¹B—SiCmaterial.

To produce the precursor formulation of the ¹¹B—SiC material, theceramic material may be doped with the ¹¹B compound and densified, suchas by heating the ceramic material in an environment or atmosphere thatincludes the ¹¹B compound. The ceramic material and the ¹¹B compound maybe heated at a temperature sufficient to convert the ceramic material toa polycrystalline SiC material. While the ¹¹B compound is in a volatilestate during the doping and densification, the ¹¹B compound may be asolid, a liquid, or a gas at room temperature. If the ¹¹B compound is asolid or a liquid at room temperature, the ¹¹B compound may be placed ina furnace or other heat source with the ceramic material. The ceramicmaterial and the ¹¹B compound may be mixed together or placed separatelyin the furnace and volatilized under the heat of the furnace so that theceramic material is doped with the ¹¹B compound. Alternatively, the ¹¹Bcompound in a solid or liquid state may be volatilized outside thefurnace and introduced to the furnace in the vaporous or gaseous form.If the ¹¹B compound is in a gaseous state at room temperature, the ¹¹Bcompound may be flowed over the ceramic material in the furnace. The ¹¹Bcompound may be used in the furnace neat, diluted in a carrier gas(e.g., an inert gas, such as argon, helium, etc.), or added under avacuum.

The ceramic material may be heated in the presence of the ¹¹B compoundat a temperature sufficient to convert the ceramic material to the¹¹B—SiC material. During the heating, the ¹¹B compound may diffuse intothe ceramic material. The temperature needed to densify the ceramicmaterial may be greater than approximately 1400° C., such as fromapproximately 1600° C. to approximately 2200° C. In one embodiment, thetemperature ranges from approximately 1700° C. to approximately 2000° C.The temperature used to convert the ceramic material to the ¹¹B—SiCmaterial should be at least equivalent to the temperature expected inany subsequent processing and/or the final utility of the ¹¹B—SiCmaterial. As the ceramic material is heated, volatile by-products mayform and should be released from the ceramic material. A rate at whichthe ceramic material and the ¹¹B compound are heated and a hold time(hold) at a maximum temperature during the heating may be adjusted aslong as the heating rate and the hold time enable the ¹¹B compound todiffuse into the ceramic material and the volatile by-products toevolve. For the sake of example only, the heating rate may range fromapproximately 1° C./minute to approximately 50° C./minute, with eitherno hold or a hold time of up to approximately several hours. However, atotal thermal exposure of the ceramic material, which depends on theheating rate, maximum temperature, and the time at the maximumtemperature, may affect characteristics and properties of the ¹¹B—SiCmaterial, such as modulus and grain growth. For example, the propertiesof the ¹¹B—SiC material may be optimized after exposure that ranges fromapproximately 5 minutes to approximately 10 minutes in the case of 10 μmdiameter fibers, which may equilibrate quickly. However, longer exposuretimes, on the order of many hours, may be used for a ¹¹B—SiC materialhaving a more substantial diameter.

The ceramic material may be heated with the ¹¹B compound for an amountof time sufficient to achieve the desired densification of the ceramicmaterial. For instance, the ceramic material may be exposed to the ¹¹Bcompound during the entire heat treatment (to convert the ceramicmaterial to the ¹¹B—SiC material) after the ceramic material begins todecompose or densify. Alternatively, the ceramic material may bemaintained at a temperature at or above its decomposition ordensification temperature in an atmosphere that includes the ¹¹Bcompound for an amount of time sufficient to enable the ¹¹B compound toincorporate into the ceramic material. The ceramic material may then befurther heated in the absence of the ¹¹B compound to complete thedensification process. Without being bound to a theory, the ¹¹B in the¹¹B compound may limit grain growth of the ceramic material and aid indensifying the ceramic material, which is believed to decrease porosityof the ¹¹B—SiC material. The ¹¹B compound may be incorporated into aconventional fiber manufacturing approach and the production of the¹¹B—SiC material may be run in batches or on a continuous productionline.

If the ceramic material to be used in the precursor formulation includesoxygen, the ceramic material may, optionally, be deoxygenated beforedoping and densification. For instance, the ceramic material may beheated to a temperature greater than or equal to approximately 1300° C.to remove the oxygen, such as a temperature that ranges fromapproximately 1300° C. to approximately 1600° C. The oxygen mayvolatilize out of the ceramic material as silicon oxide (“SiO”) orcarbon monoxide (“CO”). By removing the oxygen, the ceramic materialused in the precursor formulation may not be limited to a ceramicmaterial having a low oxygen content, which expands the types of ceramicmaterials that may be used. After deoxygenating the ceramic material,the precursor formulation may be doped and densified as described above.

After doping and densifying the precursor formulation, the resulting¹¹B—SiC material may have at least approximately 0.1 wt % of the ¹¹Bcompound incorporated therein. For instance, the ¹¹B—SiC material mayinclude from at least approximately 0.1 wt % of the ¹¹B compound toapproximately 4 wt % of the ¹¹B compound, such as from approximately 0.3wt % of the ¹¹B compound to approximately 2.5 wt % of the ¹¹B compound.

The ¹¹B—SiC material may have substantially similar properties to SiCfibers produced from a formulation that includes a ¹⁰B compound as thesintering aid. For instance, the ¹¹B—SiC material may have propertiesthat are substantially similar to those of SYLRAMIC® fibers. The ¹¹B—SiCmaterial may be high-strength, heat resistant, and corrosion resistant.In addition, the ¹¹B—SiC material may have at least 75% crystallinityand a density of at least approximately 2.9 g/cm³. Any oxygen ornitrogen that was initially present in the ceramic material may beabsent in the ¹¹B—SiC material, producing a ¹¹B—SiC material having alow residual oxygen content or a low residual nitrogen content. The¹¹B—SiC material may also have an average grain size of less thanapproximately 1 μm, such as less than approximately 0.5 μm or less thanapproximately 0.2 μm. The diameter of the ¹¹B—SiC material may rangefrom approximately 5 μm to approximately 20 μm, depending on thediameter of the ceramic fibers initially used in the precursorformulation. In addition, the ¹¹B—SiC material may have a tensilestrength of at least approximately 2,070 MPa, such as at leastapproximately 2,760 MPa. The ¹¹B—SiC material may also have an elasticmodulus that is greater than or equal to approximately 275 GPa, such asan elastic modulus that ranges from approximately 323 GPa toapproximately 483 GPa.

After densifying, a sizing may, optionally, be applied to the ¹¹B—SiCmaterial, as known in the art. The ¹¹B—SiC material may then be woundonto spools or otherwise stored until the ¹¹B—SiC material is to beshaped or produced into a product or component. For the sake of exampleonly, the ¹¹B—SiC material may be formed into tows (yarns) that includeapproximately 800 filaments. The tows may be continuous filaments of the¹¹B—SiC material and may have a diameter of approximately 10 μm and alength of approximately 1000 m. The tows may be woven into fabrics andformed into components, such as nuclear reactor components. The ¹¹B—SiCmaterial may be formed into the nuclear reactor component in the samemanner as conventional SiC fibers. Since methods of forming nuclearreactor components from conventional SiC materials are known in the art,these methods are not discussed in detail herein.

The ¹¹B—SiC material may be formed into a component for use in a nuclearreactor or in other nuclear application. Examples of components include,but are not limited to, control rods, control rod guides, fuel cladding,core support pedestals, reactor core blocks, upper core gas plenum,interior insulation covers, hot ducts, heat exchangers, or combinationsthereof. A nuclear reactor 2 is schematically illustrated in FIG. 1. Thenuclear reactor 2 includes control rods 4, a reactor core 6, a reflector8, a coolant 10, a pump 12, a heat exchanger 14, and a heat sink 16. Thecontrol rods 4 or the heat exchanger 14 may be formed from the ¹¹B—SiCmaterial. As known in the art, the nuclear reactor 2 may be connected toa hydrogen production plant 18. A close-up view of a control rod 4 isshown in FIG. 2. The control rod 4 includes an upper tie plate 20, auranium dioxide pellet 22, a water rod 24, and a lower tie plate 26.FIG. 3 illustrates a heat exchanger 14 formed from the ¹¹B—SiC material.FIG. 4 shows fuel cladding 28 formed from the ¹¹B—SiC material.

The component formed from the ¹¹B—SiC material may be heat resistant(able to withstand temperatures of greater than or equal toapproximately 1400° C.) and have a high resistance to nuclearirradiation. When irradiated, the component may not undergo outgassingor degradation since ¹¹B is more stable to irradiation than ¹⁰B and doesnot undergo fission. As such, the component formed from the ¹¹B—SiCmaterial may be used in a high temperature, corrosive, irradiativeenvironment. Using ¹¹B instead of ¹⁰B also does not significantlyincrease the expense of the ¹¹B—SiC material because the cost of ¹¹B isonly slightly greater than that of ¹⁰B. Additionally, since ¹¹B ischemically identical to ¹⁰B, special equipment or additional processingsteps are not needed.

Before shaping or producing the ¹¹B—SiC material into the product orcomponent, the ¹¹B—SiC material may, optionally, be subject to an insitu boron nitride (“i¹¹BN”) treatment. The ¹¹B—SiC material may beheated in a nitrogen atmosphere, enabling an excess of the ¹¹B compoundthat remains at a surface of the ¹¹B—SiC material to react with thenitrogen. The ¹¹B—SiC material may be heated at a temperature thatranges from approximately 1750° C. to approximately 1950° C., such as ata temperature of approximately 1800° C. The ¹¹B compound may react withthe nitrogen to form a ¹¹BN layer on the surface of the ¹¹B—SiCmaterial. The ¹¹BN layer is oxidation resistant and provides anenvironmentally durable surface and a physical barrier to the ¹¹B—SiCmaterial. The remainder of the ¹¹B compound in the ¹¹B—SiC material mayform grain boundary precipitates, specifically as titanium diboride(“TiB₂”). Removing the excess ¹¹B compound may maintain the high tensilestrength of the ¹¹B—SiC material and improves its creep resistance,electrical conductivity, and thermal conductivity. The treated, ¹¹B—SiCmaterial may also have improved rupture strength at high temperatures inair.

The following examples serve to explain embodiments of the ¹¹B—SiCmaterial in more detail. These examples are not to be construed as beingexhaustive or exclusive as to the scope of the invention.

Examples Example 1 Formation of ¹¹B-Doped SiC Fibers

TYRANNO® Lox M fibers were obtained from Ube Industries, Ltd. and solid¹¹B₂O₃ was obtained from EaglePicher, Inc. The TYRANNO® Lox M fibers arepolymer-derived, Si—Ti—C—O fibers having reduced oxygen content. Todeoxygenate the TYRANNO® Lox M fibers in a batch operation, 800 g ofTYRANNO® Lox M fibers were placed onto four Grafoil spool packages andplaced in a furnace with 108 g of ¹¹B₂O₃ powder in several Grafoil boatsin flowing argon at approximately 1600° C. for approximately 5 hours.The TYRANNO® Lox M fibers and the ¹¹B₂O₃ powder were then heated atapproximately 1650° C. for approximately 1.5 hours to enable doping ofthe TYRANNO® Lox M fibers with boron. The TYRANNO® Lox M fibers lostapproximately 20% of their weight during the doping operation. The fiberspool was then removed from the furnace.

In a second, separate operation, the doped fibers described above weredensified in a continuous spool-to-spool operation by exposing the dopedTYRANNO® Lox M fibers to a temperature of approximately 1650° C. inflowing argon for approximately 15 seconds. Upon densification, thedoped TYRANNO® Lox M fibers were converted to ¹¹B—SiC fibers. The¹¹B—SiC fibers lost no weight during this second operation. At thisstage, the fibers had a typical filament diameter of 8.5 μm, a densityof 3.14 g/cm³, a tensile strength of 2714 MPa and an elastic modulus of289 GPa.

In a third, separate operation, a sizing was placed on the ¹¹B—SiCfibers by applying a 0.5 wt % aqueous solution of polyvinylalcohol at alevel of 0.1 wt %. The ¹¹B—SiC fibers were then cured at 300° C.,producing continuous ¹¹B—SiC fibers approximately 10 μm in diameter andapproximately 1000 meters in length. The ¹¹B—SiC fibers were wound ontospools for storage as tows (yarn) of 800 filaments.

Example 2 ¹¹B-Doped SiC Fibers Exposed to iBN Treatment

Approximately 800 g of ¹¹B—SiC fibers from Example 1 were wound onto aGrafoil spool and placed into a furnace. The furnace was heated toapproximately 1850° C. for approximately 1.5 hours in a controlledflowing nitrogen gas. During this process, the nitrogen gas diffusedinto the SiC fiber surface and reacted with the ¹¹B that was present,forming a thin, ¹¹BN-rich, in-situ layer at the near fiber surface.Auger analysis showed that the ¹¹BN-rich, in-situ layer wasapproximately 200 nm in thickness. The resulting i¹¹BN SiC fibers had adensity of 3.14g/cm³, a tensile strength of 2411 MPa and an elasticmodulus of 324 GPa.

Properties of the ¹¹B—SiC fibers described in Example 1 and the i¹¹BNSiC fibers described in Example 2 are shown in Table 1. For comparison,the properties of ¹⁰B-doped SiC fibers (SYLRAMIC® fibers, commerciallyavailable from COI Ceramics, Inc.) are also provided. These propertieswere determined by conventional techniques.

¹¹B-Doped SiC ¹¹B-Doped Fibers Exposed to ¹⁰B-Doped SiC Property SiCFibers iBN treatment Fibers Denier (g/9000 m) 1605 1605 1475-1750Density (g/cm³) 3.14 3.14 3.12 Diameter (μm) 8-10 8-10  8-10 Oxygen (wt%) 0.02 0.01 0.02 Tensile Strength 2700 2600 >2067 (MPa) Tensile Modulus310 290 >276 (GPa)

While the invention may be susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and have been described in detail herein.However, it should be understood that the invention is not intended tobe limited to the particular forms disclosed. Rather, the invention isto cover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the followingappended claims.

1. A method of producing a silicon carbide material, comprising:converting a ceramic material to a silicon carbide material in thepresence of a boron-11 compound consisting of boron-11 oxide.
 2. Themethod of claim 1, wherein converting a ceramic material to a siliconcarbide material in the presence of a boron-11 compound consisting ofboron-11 oxide comprises heating the ceramic material in an environmentincluding the boron-11 compound.
 3. (canceled)
 4. The method of claim 1,wherein converting a ceramic material to a silicon carbide material inthe presence of a boron-11 compound comprises reacting a boron-11containing material with an oxidizing agent in situ to form the boron-11compound.
 5. The method of claim 4, wherein reacting a boron-11containing material with an oxidizing agent in situ to form the boron-11compound comprises reacting the boron-11 containing material and theoxidizing agent at a temperature ranging from approximately 1300° C. toapproximately 1600° C.
 6. The method of claim 4, wherein reacting aboron-11 containing material with an oxidizing agent in situ to form theboron-11 compound comprises reacting a boron-11 containing materialselected from the group consisting of boron carbide, boron, boronsuboxide, and mixtures thereof with an oxidizing agent selected from thegroup consisting of carbon dioxide, carbon monoxide, oxygen, andmixtures thereof to form the boron-11 compound.
 7. The method of claim1, wherein converting a ceramic material to a silicon carbide materialin the presence of a boron-11 compound consisting of boron-11 oxidecomprises heating the ceramic material and the boron-11 compound at atemperature sufficient to convert the ceramic material to apolycrystalline SiC material.
 8. The method of claim 1, whereinconverting a ceramic material to a silicon carbide material in thepresence of a boron-11 compound consisting of boron-11 oxide comprisesheating a ceramic material comprising silicon and carbon in nearstoichiometric amounts or in carbon-rich amounts with the boron-11compound.
 9. The method of claim 8, wherein the ceramic material furthercomprises oxygen, nitrogen, titanium, zirconium, aluminum, or mixturesthereof. 10-11. (canceled)
 12. The method of claim 1, wherein convertinga ceramic material to a silicon carbide material in the presence of aboron-11 compound consisting of boron-11 oxide comprises heating theceramic material and the boron-11 compound to a temperature greater thanapproximately 1400° C.
 13. The method of claim 1, wherein converting aceramic material to a silicon carbide material in the presence of aboron-11 compound consisting of boron-11 oxide comprises heating theceramic material and the boron-11 compound to a temperature ranging fromapproximately 1700° C. to approximately 2000° C.
 14. The method of claim1, wherein converting a ceramic material to a silicon carbide materialin the presence of a boron-11 compound consisting of boron-11 oxidecomprises heating the ceramic material and the boron-11 compound for asufficient amount of time for the boron-11 compound to diffuse into theceramic material and for volatile by-products to be released.
 15. Themethod of claim 1, further comprising heating the silicon carbidematerial in a nitrogen atmosphere to form a layer of boron-11 nitride ona surface of the silicon carbide material.
 16. The method of claim 9,further comprising deoxygenating a ceramic material comprising silicon,carbon, and oxygen before converting the ceramic material to the siliconcarbide material.
 17. The method of claim 16, wherein deoxygenating aceramic material before converting the ceramic material to the siliconcarbide material comprises heating the ceramic material to a temperaturegreater than or equal to approximately 1300° C.
 18. A material suitablefor use in a nuclear reactor component, the material comprising: asilicon carbide material; and a reaction product of a boron-11 compoundand a precursor ceramic material of the silicon carbide material, theboron-11 compound consisting of a boron-11 isotope of boron oxide. 19.The material of claim 18, wherein the silicon carbide material comprisesa polycrystalline silicon carbide material.
 20. The material of claim18, wherein the silicon carbide material comprises silicon carbidefibers, a silicon carbide body, a silicon carbide ceramic, a siliconcarbide coating, a silicon carbide thin substrate, or a porous siliconcarbide ceramic.
 21. The material of claim 18, further comprising anin-situ layer of boron nitride on a surface of the material, the boronnitride consisting of boron-11 nitride.
 22. The material of claim 18,wherein the precursor ceramic material comprises silicon, carbon, andoxygen.
 23. The material of claim 22, wherein the precursor ceramicmaterial further comprises titanium.
 24. The material of claim 18,wherein the reaction product of the boron-11 compound and the precursorceramic material of the silicon carbide material comprises titaniumdiboride, the titanium diboride consisting of a boron-11 isotope oftitanium diboride.
 25. A nuclear reactor component incorporating amaterial comprising a silicon carbide material and a reaction product ofa boron-11 compound and a precursor ceramic material of the siliconcarbide material, the boron-11 compound consisting of a boron-11 isotopeof boron oxide.
 26. The nuclear reactor component of claim 25, whereinthe nuclear reactor component is configured as at least one of a controlrod, a control rod guide, fuel cladding, a core support pedestal, areactor core block, an upper core gas plenum, an interior insulationcover, a hot duct, and a heat exchanger and assemblies thereof.